The whole reason for the Salem witch trials, at least as portrayed in Arthur Miller's The Crucible, is that some young girls were in the forest dancing and dabbling in some witchcraft-like behaviors and were seen by the Reverend Parris. Because they are afraid of being punished, probably by a humiliating and painful public whipping, the girls begin an elaborate farce which the adults around them are all too willing to believe.
Betty is one of those girls; even worse, her father is the one who caught them in the forest last night. This morning she is lying on her bed and does not (will not) move despite all of the commotion and attention which is centered around her.
Betty screams several times in act one of this play. The first time is when she is alone with the girls and Abigail frightens and practically beats Betty into responding to her demands. Betty first tries to jump out the window and then she starts rather hysterically accusing Abigail of doing worse things than the rest of them did last night. After Abigail hits her again, Betty is more subdued. Clearly Betty is terrified about what her father would do if he knew the truth about last night's activities, but she is also terrified by her cousin, Abigail.
At the end of the act, Betty cries out again; this time she does so with relief because she sees that Abigail has found a way for all of the girls to avoid punishment. When Abigail senses that Reverend Hale and her uncle, Parris, are inclined to believe in witchcraft (Hale in earnest, Paris to avoid bringing trouble into his house), she begins the charade and the other girls follow. Betty joins the throng of voices:
Betty, calling out hysterically and with great relief: I saw Martha Bellows with the Devil!
Betty does not say much in this act; when she does speak it is often a kind of screaming out of fear from either Abigail or her father.